Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
1045 PM CDT Sat Sep 20 2014
Short term...(this evening through sunday)
issued at 255 PM CDT Sat Sep 20 2014
Short term concerns deal squarely with the severe weather threat
through early this evening. A broken line of storms formed over
nodak earlier today and has organized somewhat as it pushed into
northwest Wisconsin early this afternoon. Not too much severe
weather being reported at this time...mainly due to the overall lack of
destabilization. MLCAPE values of less than 500 j/kg are expected
to continue throughout the event. However...strong forcing with
the middle level trough and very strong winds in the middle and upper
levels /60+ knots at 500 mb and 130+ knots at 300 mb/...accompanied by
inverted v soundings will bring good downward momentum and thus a
threat for mainly damaging winds. Lack of instability and low
topped convection will limit any hail possibility. The line is
moving east/southeast around 45 knots which should reach stc around 5
PM and msp metropolitan between 6 PM and 7 PM. Very high temporal grid
resolution and categorical probability of precipitation were included with relatively
high confidence due to very good agreement amongst the convective
allowing models. A general weakening can be expected after sunset
as the line enters Wisconsin when instability becomes none.
Wrap around showers may drop south into eastern Minnesota and western WI late
this evening and overnight as the upper low pushes east. These
showers may linger into early Sunday with the cyclonic flow
continuing...but surface ridging should provide enough subsidence
by afternoon to lower the threat with time.
Long term...(sunday night through saturday)
issued at 255 PM CDT Sat Sep 20 2014
Not much going on in the long term in terms of sensible weather...
but a major pattern shift is still on tap that will bring much
above normal temperatures by late next week. As the digging trough
from tonight kicks east...middle level ridging will build into the
Great Lakes and upper Midwest early next week. The middle level ridge
will be centered over the southern rockies and will allow the cut
off low over California to be kicked northeast into the upper
Midwest by midweek. Not expecting much to be accompanied with this
system aside from a low chance of showers and thunderstorms and
cooler temperatures. The thunder threat may be overplayed a bit
considering poor lapse rates...weak forcing...and dry boundary
layer air. Did not make too many changes to the previous forecast.
Models diverge a bit beyond midweek with the Canadian and GFS
building the ridge north across The Rockies while a piece of
energy rounding this ridge is detached from an accelerating
trough over northern Canada. This wave then makes an odd dive
south or southwest across the plains into a weakness in the ridge.
The European model (ecmwf) seems to keep the feature associated with the stronger
westerlies across Canada which makes most sense. Otherwise...a
massive ridge is expected to encompass all of the central and
eastern U.S. To round off the end of the month. Several members of
the GFS ensemble...as well as the operational European model (ecmwf)...center an
anomalous 591+ dm ridge over the Great Lakes Friday and Saturday.
A set up like this would normally bring the upper Midwest a heat
wave...but it is rather difficult to get oppressive heat in late
September. Temperatures will still run 10-20 degrees above average and a
few days with highs in the 80s appear likely if the center of the
ridge does build close by.
Aviation...(for the 06z tafs through 06z Sunday night)
issued at 1045 PM CDT Sat Sep 20 2014
A small band of showers will pass between krwf and kmsp before the
start of the 06z tafs. Overnigth...cyclonic low level flow will
increase across the region in response to an upper level low
pressure system moving southeast. MVFR ceilings will become
widespread across eastern Minnesota and western WI along with light rain
or drizzle. There will also be IFR ceilings for a few hours late in
the night and Sunday morning at Keau. Ceilings at krnh and Keau
will slowly improve to VFR by Sunday afternoon with clear-scattered conditions
arriving at all sites by sunset. Northwest winds near 10 knots
through the period with gust to 18 knots on Sunday.
Kmsp...a VFR ceiling will arrive early in the taf period with a
period of MVFR ceilings likely from 14-17z. This could come an hour or
two earlier than forecast and a few sprinkles are also possible.
VFR ceiling return for the afternoon along with clearing by evening.
/Outlook for kmsp/
Monday...VFR. Winds west 5-10 kts.
Tuesday...VFR. Winds S 10 kts.
Wednesday...VFR with MVFR/shra/tsra possible. Winds S 10-15 kts.